The myoD gene converts many differentiated cell types into muscle cells. MyoD protein is a member of the basic-helix-loop-helix (bHLH) family of proteins; this 68-amino acid domain in MyoD is necessary and sufficient for myogenesis. The MyoD protein is sufficient to orchestrate the coordinated expression of most, if not all, of the skeletal myogenic program in cell types derived from all three germ layers of the embryo. As such, the regulation of Myo D expression and protein activity creates a nodal point, or master switch, that integrates the genetic and environmental influences on a cell. The expression and activity of the MyoD protein is regulated, in part, by interactions with members of a large family of proteins related to MyoD by sequence homology, the helix-loop-helix (HLH) proteins. MyoD binds cooperatively to muscle-specific enhancers and activates transcription. The helix-loop-helix (HLH) motif is responsible for dimerization, and, depending upon its dimerization partner, MyoD activity can be controlled.
MyoD is a nuclear protein, 318 amino acids in length, that binds to many muscle-specific enhancers. Although it is phosphorylated, the function of the phosphorylation has not been defined. The protein and RNA turn over rapidly. MyoD binds to a consensus DNA binding sequence that includes a CAN NTG sequence (N represents A, T, G, or C) present in most muscle-specific enhancers (1; see the appended Citations).